Thick film technology has historically been an attractive method of producing conductors, dielectrics and resistors that are rugged and reliable. The technology is well suited for economical production of short production runs. Its ability to be patterned in multilayer configurations has allowed fabrication of devices with namely high circuit density. The successive levels of conductors in the multilayer structure are separated by insulating dielectric layers and are interconnected by vias through the dielectric layers.
The multilayer approach is more expensive than a single layer approach because it requires painstaking inspection, realignment between the layers, and careful processing to avoid blistering and cracking.
The most obvious way to reduce these problems associated with multilayer production is to reduce line and space dimensions, thereby reducing the number of layers in a given structure. The problem with this approach has been the limited resolution capability of thick film screen printing, which limits the size of vias used to connect layers of circuitry to 10 to 15 mils diameter. Likewise, conductors are limited to a narrowest line width and spacing of 5 to 7 mil lines and spaces in production quantities.
Many different approaches have been tried to obtain finer pitch lines and smaller vias. Extremely fine screen mesh and improved emulsion backing have allowed line resolution of as low as four mils line/space to be obtained in limited production. Photoformable pastes have been developed that allow five mil or finer vias, and two to three mil line/space pitch. Thick film metallizations have also been patterned with photoresists and etched to produce fine line patterns and thin film conductors have been plated up to produce fine line patterns with high conductivity.
All the above approaches have associated drawbacks. For example, fine mesh screens typically lay down thinner conductor and dielectric layers than are desirable. Photoformable pastes have a larger amount of organic matter that increases shrinkage during firing and can produce dirty burnout that may render fired parts useless. Conductors produced with photoformable pastes have an undesirable edge curl that can reduce the reliability of circuits fabricated with them. The processes that require etch, photoresists or plating are lengthy, process-sensitive and expensive. Furthermore, some of the processes use solvent that is difficult to handle. However, non-photoformable means of patterning dielectric films using screen printing techniques has been shown capable of generating much smaller vias than screen printing the vias directly. Where direct screen printing of vias is capable of producing only 12 mil vias or larger in production, vias as small as four mils diameter can be patterned with very high yield by indirect patterning.
Diffusion patterning, as it is currently practiced in the industry, uses an alkaline patterning ink that is overprinted on top of the film to be patterned. The alkaline ink is then diffused into the film using heat, after which overprinted areas become removable in a washout step using water.
Current practice using acrylic polymers with acidic groups that can be ionized by overprinting using an alkaline patterning ink have the disadvantage that they are reactive with some ionic species in glasses and inorganic oxides, especially divalent oxides. Their reactivity may adversely affect the stability of printing inks. Also, some digital patterning techniques use ink jet engines with printing cartridges that do not withstand highly alkaline inks well. Clearly, the present invention is directed to a composition that includes polymers that are capable of being activated by non-alkaline inks and that do not have the reactive acid groups that will provide the basis for diffusion patterning compositions with improved properties for numerous applications.